The long range goal of this project is to combine immunocytochemical methods with light and electron microscopy to study cellular mechanisms of myelin formation, breakdown, and regeneration. The main effort is directed at mechanisms relevant to human demyelinating diseases. Nervous tissues from experimental animals and humans have been studied in the following sets of current experiments: 1) use of electron microscopic immunocytochemistry and polyclonal rabbit antisera to localize myelin-associated glycoprotein (MAG). Using post-embedding immunostaining, anti MAG reaction product was found on compact adult CNS myelin. 2) Using the same EM immunocytochemical method with a monoclonal antibody, anti MAG immunoreactivity also was localized in compact CNS Myelin. 3) Light and electron microscopic study of serially sectioned mitotic glial cells in developing white matter. The results led us to conclude that developing a oligodendroglia probably do not divide while forming myelin sheaths.